A week ago Sir Alex Ferguson was at Manchester United's academy training complex watching an Under-12s game, when he observed Rio Ferdinand making his way towards him in a buggy.

"I thought to myself: 'He must have something to tell me,'" the United manager says. "He must want to play against Chelsea." Sure enough, after a minimal amount of small talk – "He asked me the score and I had to tell him we were losing one-nil to Liverpool" – Ferdinand declared himself ready to play after two months out with a calf injury, and his manager took him at his word.

"I was thinking of playing him anyway, but it would have been a risk to have rushed him back," Ferguson says. "I was pleased I didn't need to persuade him. As soon as Rio said he was fit I was satisfied we would not be taking a gamble – he knows his own condition better than anyone else – and in the event he was fantastic for us against Chelsea. I was expecting him to tire towards the end but there was no sign even of that. He's a natural athlete, a bit like Paul McGrath used to be, and it's good to have him back. He's fresh again now as well, and I always feel that with our first-choice defence in place we have a good chance against anyone."

The case for United's defence is particularly solid in Europe. It is six years since they last lost a Champions League tie at home, and so far this season they have conceded a miserly two goals. By this stage last season, by comparison, they had let in 10, and even Barcelona, the favourites to lift another European title at Wembley in May, have seven in the debit column, as have Schalke. Internazionale have a whopping 19. Maybe defensive security was the basis for Ferguson's unusually bullish claim last week that his side have another treble within their grasp.

"The defence has done very well, although that might have a lot to do with the way we have operated our midfield in Europe this season," Ferguson says. "We have a lot of experience of playing in Europe now, and knowing how to keep possession of the ball has helped. We've found a way of playing in Europe, and I think we have the right players for the job on Tuesday. Now we have everyone fit again the defensive part is much stronger than it has been, and that is a definite bonus. At this stage of the season that is all you can ask, really. It hasn't always worked out for us this way in the past, but when you see everyone getting back in time for the important games it's fantastic."

One player who will be back, fresh and highly motivated for the Chelsea game is Wayne Rooney, goalscorer in the first leg but granted what may be termed an official rest over the weekend. "At least we know he's not going to get himself injured," Ferguson says. "He is disappointed by what has happened, as we all are – none of the players can understand it. You have to take your medicine and move on, though, and Wayne is ready to do that. The way I'm looking at the situation is to try and use it to our advantage. We have all got great motivation now, not just Wayne. I think you will see a difference in our supporters too."

With an away goal and a clean sheet from the first leg, United would appear to be in a commanding position at a fervent Old Trafford, and with Schalke their likely semi-final opponents it will now be a surprise if they do not reach a third Champions League final in four years. Not that their manager will have any truck with looking so far into the future. As far as he is concerned the tie is still finely balanced, and Chelsea far from out of it.

"We will be approaching the game in exactly the same way as we did in the last round against Marseille," he says, referring to a tie that was goalless after the first leg. "It is best to ignore the goal we already have and just go out to try and win the game. If we win the match we go through and that's the easiest way to look at it. You don't want to be playing for a draw or worrying about what happens if they score. I think Chelsea will play their usual game, building up slowly rather than setting off in a rush to score, but if we play our usual game we should be all right."

• The following correction was printed in the Guardian's Corrections and clarifications column, Monday 18 April 2011. This article said it is 'six years since Manchester United last lost a Champions League tie at home', overlooking their 0-1 defeat to Besitkas on 25 November 2009.